The present invention relates to a solid-state reference electrode for a potentiometric electrode system and, more particularly, to a miniaturized solid-state reference electrode characterized by employing an ion-sensitive membrane containing an ion-selective material as protective membrane for its inner reference solution and enlarging area of junction contacting with the sample solution, whereby operational abnormality may be checked by itself and fast activation may be achieved.
The levels of molecules of interest in liquid are useful indexes with which the liquid states can be perceived and thus, it is very important to quickly and accurately measure the molecular levels in physiological fluids such as blood, urine and the like, for the purpose of clinical analysis. Such importance is also true of domestic water, such as tap water and sewage, and industrial water and wastewater in environmental and health aspects. In addition, industrial final products, by-products, and intermediates are necessary to be analyzed as particular molecules in order to check the product quality.
Electrode systems utilizing potentiometry find numerous applications for the analysis of a broad spectrum of molecules ranging from biochemical compounds such as urea, creatinine, glucose in blood and urine to ions such as K+, Na+, NH4+, Ca2+, Mg2+, HPO42xe2x88x92, Clxe2x88x92, CO32xe2x88x92, etc., and gases, such as pH, pCO2, pC2, pNOx, pSOx, etc. in industrial materials for the clinical analysis, water examination, and technical analysis fields.
Generally, such a potentiometric electrode system consists of two electrodes: a working electrode that reacts with an interested ion to generate a potential difference; and a reference electrode that maintains a predetermined potential. In this system, the potential values measured on the working electrode are not absolute, but they show the relative values to the predetermined potential maintained by the reference electrode, in other words the potential difference between the working electrode and the reference electrode. Therefore, the development of a stable reference electrode to maintain a predetermined potential has a very great significance in the advance o the potentiometric electrode system.
The potentiometry for the analysis of clinical, liquid and industrial materials, like other assay methods, requires the samples to be unchanged after they are taken. However, the samples are likely to be contaminated during transportation or changed in quality owing to a measurement delay, resulting in inaccuracy. Thus, such analysis requires point-of-care for measurement. In particular, medical assay for clinical materials, such as blood, must solve the above problems as well as rarity of the samples because only a small amount of biomaterials from a testee, for example, a medical patient, is allowed for clinical purposes.
To meet these requirements, active research has been and continues to be directed to the miniaturization of measuring apparatuses. In this regard, an essential prerequisite is to micronize the electrode system consisting of working electrodes and reference electrodes. In fact, it is not too much to say that it is essential for the miniaturization of potentiometric electrode systems. Of the components of the potentiometric electrode system, the working electrode has been and continues to be under active research and, as a result, many methods are realized and suggested. On the other hand, the micronization of the reference electrode which is the counterpart of the working electrode seems to be very far away from success and actually, has been under scanty study and leads to the greatest obstructive factor to achieve the miniaturization of the entire potentiometric electrode system.
As reference electrodes, calomel Ag/AgCl electrodes are conventionally used while a capillary tube or a porous ceramic functions as a junction therein. However, these conventional reference electrodes are impossible to be miniaturized and manufactured on a large scale. In addition, the conventional reference electrodes suffer from high production cost. As a result, they are not suitable for the application for small or disposable analyzers.
Various attempts have been made to develop solid-state reference electrodes sufficient enough to be applied for small potentiometric electrode systems and the following two techniques have been evaluated as the most successful.
One technique is to employ liquid junctions, instead of solid junctions, such as conventional capillary tubes or porous materials, in which two flow systems are constructed: one flow system for an electrode of an insoluble metal salt, such as AgCl, the other for a working electrode. These two flow systems are designed to meet at a point at which an electrode junction is formed. Electrodes of insoluble metal salts are reactive to particular ions (AgCl to Cl ions). Flowing a reference solution comprising the particular ions (in case of AgCl, NaCl solution) with constant concentration to the flow system have the insoluble metal salt electrode kept as a constant potential (reference electrode system). On the other hand, when calibrants or sample solutions have flowed through the flow system in which a working electrode is established, the working electrode reacts with the particular ion and a potential change is measured as the potential difference relative to the reference electrode system. The mentioned reference electrode, even if it is relatively simple in the construction, suffers from the disadvantages of requiring an additional reference solution and being high in maintenance cost, further the flow may not be easily controlled because of its solution flow division into two.
The other alternative provides a structure in which a reference solution is formed into a hydrogel on an insoluble metal salt such as AgCl and a polymer protective membrane is coated over all areas of the hydrogel, except for a window (junction) at which the hydrogel make contact with a sample solution. Saturation of the inner reference solution with a salt such as KCl makes the potential of the insoluble metal salt constant, but the saturated salt gradually comes out through the junction, changing the salt concentration of the hydrogel and thus, the potential of the insoluble metal salt electrode. Further, the insoluble metal salt electrode plays a role as a reference electrode for several minutes before the occurrence of a potential change. This type of a reference electrode enjoys advantages of being easy to construct and control, therefore, no additional constructions are required. However, this conventional reference electrode has some problems in that the activation time of hydrogel for the normal operation, that is, the time for the hydrogel to absorb moisture from the sample for maintaining the stabilization, is relatively long. Further, the small window that serves as a junction is apt to be constructively inaccurate or is frequently clogged by impurities such as floating materials of samples. Moreover, the abnormality of the reference electrode caused by the above problems cannot he detected without operating the reference electrode, so that the measured values are poor in reliability.
The intensive research on potentiometric electrode systems have been repeated to overcome the above problems of conventional reference electrodes, resulting in the discovery of the employment of an ion-selective material into the protective membrane of the inner reference solution, together with an enlarged reference electrode junction in contact with a sample solution, enables the determination as to whether the reference electrode is under operational disorder such as contamination as well as reduction of the contamination at the junction and guarantees the reference electrode to be fast activated in a solution.
Therefore, it is an object of the present invention to provide a solid-state reference electrode with a self-diagnostic function, which is improved in junction contamination and can be activated within a short period of time.
According to the present invention, there is provided a planar-type solid-state reference electrode comprising a substrate; a metal layer; an insoluble metal salt layer; an insulating film for insulating the metal layer from an aqueous solution; a hydrogel serving as an inner reference solution; an enlarged junction on the hydrogel, at which the hydrogel is in contact with a sample solution; an ion-sensitive, protective membrane formed over all surface areas of the hydrogel, except for the junction, to separate the inner reference solution from the outer sample solution and act as an ion-sensing membrane upon the contamination of the junction.